1. Field of the Invention
The present invention relates to an electroluminescent display element and, particularly, relates to a multi-color electroluminescent display element utilizing a color filter.
2. Related Art
A color of light emitted from an electroluminescent element is yellowish orange when ZnS (zinc sulfide) is used as a host material and Mn (manganese) serving as a luminescent center is doped thereto while, it is green when Tb (terbium) serving as a luminescent center is doped thereto.
Japanese Patent Application Laid-Open No. Hei.2-112195 discloses an electroluminescent element wherein a luminescent layer of ZnS:Mn using ZnS as a host material and having Mn, serving as a luminescent center, doped thereto and a luminescent layer of ZnS:Tb using ZnS as a host material and having Tb, serving as a luminescent center, doped thereto are laminated one on the other; and red-color and green-color filters are provided on the resultant structure, thereby making emission of multi-color light.
In the multi-color electroluminescent display element using such color filters, loss of transmission of light due to the color filters is large.
The Assignee of this application has filed U.S. patent application Ser. No. 08/598529, disclosing an electroluminescent element in which a luminance of emitted light of green color was increased by providing only a red color filter. In the electroluminescent element, as shown in FIG. 20, a luminescent layer 5 of ZnS:Tb is laminated on a luminescent layer 4 of ZnS:Mn formed in a predetermined pattern, and red color filters 8 are deposited in correspondence to laminated portions of the luminescent layer 4 of ZnS:Mn and the luminescent layer 5 of ZnS:Tb. Light of green color is emitted from single-layer portions consisting of the luminescent layer 5 of ZnS:Tb and light of red color is emitted from the laminated portions via the red color filters 8. It is to be noted that numbers in the figure corresponds to the numbers used to describe preferred embodiments later.
The inventors have gone ahead with studies on an electroluminescent element which has such a color filter and emits lights of multi colors. As a result, they have found out that light from a luminescent layer located below a color filter leaks out of the color filter without passing therethrough, thereby deteriorating purity of color.
For example, in the electroluminescent element illustrated in FIG. 20, light of yellowish orange color emitted from the luminescent layers 4 of ZnS:Mn leaks out of the redcolor filters 8 as shown by arrows in the figure. Accordingly, light of red color passing through the red-color filters 8 and light of yellowish orange leaking out thereof are mixed, thereby deteriorating purity of color.